Double metal cyanide (DMC) catalyst complexes are known to be useful catalysts for polymerizing 1,2-propylene oxide. DMC catalyst complexes have certain advantages over the more traditional potassium hydroxide catalysts. DMC catalyst complexes in most cases do not need to be removed from the polymerized product. This represents a very significant savings in manufacturing costs. In addition, DMC catalyst complexes produce a product that has a low level of monofunctional impurities.
A disadvantage of DMC catalyst complexes is they tend to produce a quantity of undesirably high molecular weight material, which often consists of two distinct fractions. One fraction has molecular weights from about 20% greater than the peak molecular weight (as determined by gel permeation chromatography), up to about 30,000; this fraction may constitute 1% to 15% or more of the mass of the polymer. The other fraction is very high molecular weight material (50,000 or more and often 100,000 or more g/mol) that is present in very small amounts but is nonetheless very significant. This very high molecular weight fraction is a potent surfactant, and is known to interfere with the production of polyurethane foam from the polyether polyol product.
One way to reduce amount of high molecular weight material is by performing the polymerization in a semi-batch method, in which a starter compound and propylene oxide are continuously added to a reactor that contains the activated catalyst. Such a method is described in U.S. Pat. Nos. 5,777,177 and 6,077,978. Unfortunately, this approach leads to the opposite problem, that is, the creation of a significant fraction of low molecular weight material. This low molecular weight material may constitute 5 to 15% or more of the total mass of the product. The low molecular weight material includes polyethers that have molecular weights 65% or less of the peak molecular weight of the material.
The low molecular weight material causes significant difficulties in some applications. One such application is the production of polyurethane foams using as the blowing agent a mixture of water and certain physical blowing agents such as cyclopentane. It is difficult to form a homogeneous mixture of the polyol, water and physical blowing agent, in part at least because the low molecular weight material tends to coalesce with the water and form a separate phase. This problem is not seen when the polyether polyol is made using potassium hydroxide catalysts. In that case, the polyether polyol, water and physical blowing agents form a homogeneous mixture, and so these components can be mixed to form a formulated “B-side” composition that can be stored and transported. Because of the compatibility problem, corresponding mixtures in which the polyether polyol is a DMC-based polyol tend to phase-separate.